This application claims Paris Convention priority of DE 101 18 835.8 filed on Apr. 17, 2001 the complete disclosure of which is hereby incorporated by reference.
The invention concerns an RF (=radio frequency) receiver coil arrangement for receiving measuring signals from a measuring sample in the measuring volume of an NMR (=nuclear magnetic resonance) spectrometer, comprising an RF resonator having superconductive, inductively and capacitively acting conductive structures, which form resonant circuits, which are disposed on planar substrate elements, and which are disposed externally around the measuring sample.
An arrangement of this type is known from U.S. Pat. No. 5,585,723.
NMR is a very powerful and exact method for analyzing the structure of chemical compounds. Unfortunately however, it is not very sensitive. For this reason, it is of central interest in NMR to provide resonators which have as high a detection sensitivity as possible, i.e. as high a signal to noise (S/N) ratio as possible. The use of cooled and in particular superconducting radio frequency resonators permits the losses in the resonator to be kept very small thereby considerably increasing the sensitivity.
HTS material is currently the most suitable superconductor. This material has a high transition temperature and, compared to other superconductors, is very insensitive to static magnetic fields. These advantageous properties are obtained only when the superconductor is made from very thin epitaxial layers which are formed on oriented monocrystalline substrates. Since such substrates are normally only available as flat plates, the geometric shape of the resonators is constrained to be in the form of flat plates, thereby considerably limiting the possible geometric arrangements.
The substance to be measured is normally a liquid enclosed in a measuring tube usually at room temperature, which is separated from the cold NMR resonator (at approximately 20K) by an intermediate tube and a vacuum chamber.
Such arrangements are known e.g. from U.S. Pat. No. 5,585,723. Superconducting receiving resonators have the following principal problems. The static magnetization produced by the superconductor can cause field inhomogeneities which substantially prevent the generation of narrow lines in high-resolution NMR. Moreover, the finite critical current in the superconductor limits the maximum coil current for the transmitter pulse, thereby complicating or impeding short pulse widths for a predetermined NMR flip angle.
As mentioned above, the sensitivity of the resonator, i.e. its S/N ratio, plays a primary role in NMR. It depends on the volume integral of the term B1(x,y,z)/sqrt(P), wherein B1 designates the high frequency field which is produced by the resonator at the location x, y, z when provided with power P. If the contributions of the currents in the conductors all have identical values, the conventional equation P=Rxc2x7I2 can be inserted for the power P, wherein the term in the volume integral becomes B1(x,y,z)/Ixc2x7sqrt(R). This means that the larger the B1 field produced by the resonator per unit current I and the smaller the overall loss resistance R of all conducting paths, the higher the sensitivity.
The NMR resonator must not only have high sensitivity but its conductors must also carry large currents to permit production of as high B1 fields as possible during the transmitting phase. High B1 fields permit short transmitter pulses for obtaining a given desired NMR flip angle. Such short transmitter pulses are highly desirable in NMR for various reasons which will not be discussed herein. Although resonator sensitivity allows optimum conversion of the current into a B1 field, this is not sufficient in and of itself. The current should also be as high as possible. This, in turn, depends i.a. on the critical current of the superconductor and on the width of the conductor. The geometry of the resonator should therefore be such that those conductors which are mainly responsible for producing the B1 field are as broad as possible and can carry the full critical current.
A further parameter which is of major importance to the NMR spectrum is the resolution. The resolution substantially depends on the relative line width which can be obtained for the NMR signal which, in turn, depends on the homogeneity of the stationary magnetic field H0 in the active region of the measuring sample. Since the static magnetism of the superconducting material in the resonator can have a strong negative effect on the homogeneity of the H0 field due to the short separation of the superconductor from the measuring sample, the surfaces of the individual superconducting conducting elements must be specially designed. One solution of prior art provides for the subdivision of all conductors into narrow strips to minimize the influence of magnetism to the greatest possible extent.
The Q of the resonator is also an important value which can influence the NMR spectrum. It should not be too high in order to keep transient and decay processes, mainly caused by the transmitter pulses, short for minimizing undesired artifacts in the NMR spectrum. Since the Q value is equal to the ratio xcfx89L/R, i.e. the impedance of the overall inductance of the resonator, divided by the overall resistance of all conducting paths, the inductance L of the resonator should be as small as possible for a given resistance R. This can be obtained by keeping only those inductances which are mainly responsible for generating the B1 field in the measuring sample, and by reducing or compensating for all others to the extent possible.
Moreover, the resonator should permit a second resonator to be mounted as close to the measuring sample as possible such that its magnetic field is orthogonal to that of the first for minimizing magnetic coupling between the two resonators. The resonance frequencies of the two resonators can either be the same or different. Of these two options, the latter is usually chosen and the second resonator serves to excite and/or receive signals from a second type of nucleus. Orthogonal resonator arrangements of this type are often used in NMR for special experiments.
In particular, two types of resonators are known, i.e. the coplanar Helmholtz resonator (see U.S. Pat. No. 5,585,723) and the hybrid resonator (see U.S. Pat. No. 6,121,776).
Coplanar Helmholtz Resonator
This resonator consists of two planar resonant circuits in a Helmholtz arrangement (see FIGS. 7 and 8) which are tuned to the same resonance frequency f0 and which produce two resonant modes due to their strong inductive coupling, one below and the other above f0. The mode with the lower resonance frequency is the only one which can produce a homogeneous B1 field in the measuring volume. The other mode produces a highly inhomogeneous field and can therefore not be used as an NMR detector.
The opening angle xcex1 of the resonant circuit arrangement is usually selected to be 120xc2x0 (see FIG. 7) to obtain the best homogeneity of the B1 field.
Since the coil consists exclusively of superconducting material, the losses are very small and the sensitivity could be very high were it not for other impairing factors. In particular, the planar geometry prevents the conducting paths from being placed in the direct vicinity of the measuring sample, therefore producing a correspondingly smaller B1 field at the location of the measuring sample. The sensitivity is consequently reduced.
The structure of each of the two resonant circuits is shown in FIG. 9. All conductors perform a dual function, namely generation of both the resonance inductance and the resonance capacitance of the resonant circuit. It can be shown that the largest possible average current through such conductors is only half of the critical current for the superconductor should the individual conductors have the same widths. The maximum B1 field which can therefore be obtained is likewise only half the size of the highest theoretically possible field. However, one would like to operate at the full critical current.
If, as is usually the case, the same resonator is also used for transmitting the excitation pulses, it is very important to obtain as high a current in the conductors as possible and therefore a maximum B1 field since this determines the pulse width needed to obtain a desired NMR flip angle. Moreover, that pulse width should be as small as possible. The B1 field is halved by the effect described in the previous paragraph and is further reduced by the relatively large distance between the conductors and the measuring sample.
The large dimensions of the two resonant circuits caused by the planar geometry also causes the inductance of this resonator to be many times greater than the minimum possible inductance. These resonators have Q values of 30,000 and more. This leads to transient and decay processes, mainly caused by the transmitter pulse, which are of excessive time duration and which therefore cause distortions in the base line and generate undesired artifacts in the NMR spectrum.
The large size of the resonator has the additional consequence that there is little free space for providing additional resonators in the vicinity of the measuring sample. Moreover, the resonator conductors are located close to the shielding and are magnetically and electrically coupled thereto, thereby causing additional losses.
Hybrid resonator
This resonator is preferably designed as a birdcage resonator. It is constructed from massive superconducting conducting paths disposed in an optimized fashion around, and parallel to the axis of, the measuring sample. These conducting paths are capacitively connected to each other at both ends of the conducting paths via a normal-conducting, tubular element.
Since the conducting paths are disposed in close proximity to the measuring sample and act nearly purely inductively, without having to serve a capacitive function, they can carry the full critical current. These two factors allow the maximum B1 field which can be generated in the sample volume to assume an optimum value. This resonator type also allows correspondingly optimum short pulse widths for a desired NMR flip angle.
Compared with the Helmholtz resonator which has purely superconducting resonant circuits, the overall loss resistance R of the hybrid resonator is slightly higher and its overall inductance L is much smaller, since the conducting paths closely surround the measuring sample. This produces a quality factor Q=xcfx89L/R which is considerably smaller and therefore better than that of the Helmholtz resonator.
Although one might expect the higher overall losses of the hybrid resonator to produce a worse S/N ratio, this effect is compensated, or even overcompensated, by the fact that the conducting paths are located much closer to the measuring sample and therefore produce a much larger B1 field. In certain cases, S/N ratios which are higher than those of a Helmholtz resonator can be obtained.
The very compact construction permits optimum installation possibilities, in particular for measuring samples having large diameters, with which the amount of remaining free space can be very limited. However, for smaller diameters of the measuring sample, the somewhat complicated construction has structural limits.
In summary, the NMR resonator should have the following five important features:
1) It should have optimum sensitivity.
2) It should generate the desired NMR flip angles during the transmitting phase with as short pulse times as possible.
3) Its influence on the homogeneity of the stationary Ho field in the region of the measuring sample should be as small as possible.
4) Its Q should not be too high, such that the transient and decay processes mainly caused by the transmitter pulses can be kept short to prevent generation of excessive artifacts in the NMR spectrum.
5) Its geometrical design should be such as to provide the space required for optimum installation of one or more orthogonal resonators.
Departing therefrom, it is the underlying purpose of the present invention to present a new class of superconducting NMR resonators which satisfy the above-mentioned requirements to present an improvement over prior art.
This object is achieved in accordance with the present invention in a surprisingly simple and effective manner in that each individual current-carrying resonant circuit on the planar substrate elements generates a magnetic field in the center of the measuring volume which is parallel to the plane of the planar substrate element on which the single resonant circuit is located, wherein the deviation from parallelism should not exceed 40 degrees.
The invention presents a manner of NMR resonator construction with improved sensitivity, minimized pulse widths for a given desired NMR flip angle, minimized resonator Q and therefore small spectral artifacts, as well as optimum space for installation of additional resonators in an orthogonal direction relative to the first resonator. Nearly all of these features constitute considerable improvements over prior art.
These resonators have excellent NMR properties and are also of simple construction to permit easy assembly, even with confined geometries.
Further advantages can be extracted from the drawing and the description. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration, rather have exemplary character for describing the invention.
The invention is shown in more detail in the drawing and is further explained with reference to embodiments.